Commercial refrigeration systems are used to maintain a cooled space or a refrigerated product at a desired temperature. Refrigerated cases or rooms are commonly used by grocery stores, restaurants, warehouses, and food distributors. In order to maintain the quality of the refrigerated product while minimizing the cost of refrigeration, it is necessary to keep the temperature of the refrigerated product or case as close to the desired temperature as possible. If the temperature is allowed to rise, the quality or integrity of the product may be jeopardized. If the temperature is kept lower than necessary, energy is wasted and the already high cost of refrigerating the space is increased unnecessarily. For example, it is estimated that as much as 2% more energy is required for each degree the temperature runs below the desired temperature.
Several factors may be used to monitor and control the refrigerated space. These include suction pressure, head pressure, and the temperature of the air being discharged by the cooling system into the refrigerated area. If the desired product temperature is known, the required discharge air temperature can be determined. Alternatively, the suction pressure required in order to maintain a desired temperature may be determined by referring to a pressure-temperature chart for the particular refrigerant being used. The pressure-temperature chart provides the suction pressure that is required to be provided by the system in order to maintain the desired temperature.
The most common prior art refrigeration control systems attempt to maintain the desired temperature by monitoring and controlling the suction pressure of the compressors. Generally described, these systems define an acceptable pressure range by providing upper and lower pressure setpoints, referred to as cut-in and cut-out setpoints, respectively. When the suction pressure exceeds the cut-in setpoint, the system turns on additional compressors in order to increase the cooling capacity of the system. When the suction pressure falls below the cut-out setpoint, the systems turns off some of the compressors in order to decrease the cooling capacity of the system. As a result of using these upper and lower limits, the system includes a range or "dead band" in which the system is uncontrolled.
In order to restrict the number of compressors that are turned on or off at one time, the prior art systems establish certain predetermined combinations of compressors that are allowed. Thus, even if a system includes four or five compressors, a predetermined table is often set up in order to ensure that no more than two compressors change state at one time. These tables attempt to equalize the run time of the compressors and reduce inrush currents. Prior art systems also provide time delays in order to control the period of time between changes in compressor capacity. This reduces the wear and tear that results from starting and stopping the compressors more often than necessary. Thus, even if the suction pressure moves beyond the cut-in setpoint, the cooling capacity will not be increased until a predetermined time period has elapsed. Once this occurs, the controller will step to the next higher combination of compressors as provided in the table. Likewise, if the suction falls below the cut-out setpoint, the controller will step to the next lower combination of compressors only after the delay has elapsed. Thus, when a change in capacity is required, the amount of change is limited to the next step in the predefined table, and will be delayed until the preprogrammed delay time has elapsed.
FIG. 1 is a graph of the variations in suction pressure versus time for a prior art controller of the type described above. As can be seen, prior art controllers that use separate cut-in and cut-out setpoints have an inherent dead band problem. The system is not controlling the refrigeration system when the suction pressure is between the two setpoints. In fact, by including a time delay, the system is only controlled when the suction pressure has moved outside the setpoints for some period of time. This overshoot results in wide swings in suction pressure.
Limiting the number of compressors that can change energization state also forces the refrigeration system to go through extra steps in order to reach the necessary cooling capacity. For instance, consider a prior art controller and a refrigeration system that includes one 5 horsepower (hp) compressor and three 10 hp compressors. The predefined step table would typically allow the capacity to increase in the following steps: 5 hp, 10 hp, 15 hp, 20 hp, 25 hp, 30 hp, and 35 hp. If at system startup, the load required capacity of 35 hp, the prior art controller would go through each of the seven states with a delay in between successive states. If the delay was set to 90 seconds, it would take a minimum of 9 minutes for the controller to match the load. In addition, it would require 12 changes of energization state. Such a time delay in matching the capacity to the load wastes energy. The required changes in energization state increase the amount of wear and tear on the compressors.
As described above, the suction pressure required to maintain a desired temperature can be determined by referring to a pressure-temperature chart for a refrigerant. This is the only suction pressure that will maintain the desired product temperature in an ideal system. However, other factors such as evaporator superheat, refrigerant line sizing, and ambient temperature affect the relationship between the suction pressure and the temperature. As a result, one cannot depend on the calculated suction pressure value to be the optimum pressure. Prior art systems have attempted to address this problem by monitoring the temperature of the refrigerated space at predetermined intervals. If the refrigerated space is too cool, the cut-in and cut-out setpoints are increased by a predetermined incremental amount. Likewise, if the space is too warm, the setpoints are decreased by a predetermined incremental amount. This improved control over earlier systems. However, if the load changes quickly, such as when new products are added to the refrigerated space, the incremental adjustments to the setpoints could result in a large amount of time passing before the controller matched the capacity to the load.
Other prior refrigeration control systems have incorporated proportional-integral-derivative (PID) control algorithms. It is well known that such conventional linear controllers can be designed to achieve good results if the controlled process is stable and well-defined, and if the designer has a good mathematical model of the system. However, the refrigeration process is dynamic, non-linear, and unstable, and does not lend itself to accurate mathematical modeling. As such, it is very difficult to design and set up a satisfactory controller using conventional methods.
Therefore, there is a need in the art for a refrigeration system controller that overcomes the described disadvantages in spite of the dynamic, unstable, and non-linear nature of the refrigeration process. Such a system would be able to efficiently and accurately match the capacity of the system to the thermal load and thereby minimize the waste of energy and unnecessary wear on the system components.